1. Field of the Invention
The present invention relates to an anode active material, a method of preparing the same, and an anode, and a lithium battery comprising the material. In particular, the present invention relates to an anode active material that has excellent charge/discharge efficiency and is effective charge capacity, a method of preparing the same, and an anode, and a lithium battery comprising the material.
2. Description of the Background
Non-aqueous electrolyte rechargeable batteries that include a lithium compound as an anode, exhibit high voltage and high energy density and have been studied. Previously, lithium metal was studied because of its high power storage capacity. However, when metallic lithium is used as an anode material, lithium dendrite deposits onto the surface of metallic lithium on charging. The lithium dendrite reduces charge/discharge efficiency of the battery and may provoke a short-circuit. Also, the risk of explosion and high sensitivity to heat and shock caused by lithium's instability or high reactivity prevented metallic lithium anode batteries from commercializing.
The use of a carbon-based anode addresses some of the problems of lithium. Lithium ions present in electrolyte intercalate/deintercalate between the graphite layers of the carbon-based anode, thereby performing oxidation/reduction reactions. The carbon-based anode has solved various problems associated with lithium metal and has contributed to its popularization. However, there is the need for lightweight, low volume lithium rechargeable batteries with higher capacity for use in portable electronic devices.
A lithium battery containing a carbon-based anode has essentially lower battery capacity due to its limited lithium storage ability. For example, even for graphite with the highest crystallinity, the theoretical capacity of a LiC6 composition is about 372 mAh/g which is less than 10% of the theoretical capacity of lithium metal of 3860 mAh/g. Therefore, much research has been performed to improve the power storage capacity of a battery by introducing a metal such as lithium to anode.
Alloys such as Li—Al, Li—Pb, Li—Sn and Li—Si may provide higher electric capacity than a carbon-based material. However, when such alloys or metals are used alone, deposition of lithium dendrite occurs. Therefore, use of a suitable mixture of such alloys, metals, and carbon-based material has been researched to not only increase electric capacity but also to avoid problems such as a short circuit. However, the volume expansion coefficient on oxidation/reduction of a carbon-based material is different from that of a metal, and the metal can react with electrolyte. On charging of an anode material, the lithium ion is introduced into the anode material. The anode expands and its structure becomes more dense. Then, on discharging, the lithium ion leaves and volume of the anode decreases. If the anode contracts, a void remains for spatial gaps that are not filled due to the difference in the expansion coefficient of carbon-based material and that of the metal. Therefore, due to the electrically non-connected spatial gap, electron conduction is ineffective and the efficiency of the battery decreases. Also, a reaction between the metal and the electrolyte during charge/discharge can decrease the lifespan of electrolyte, and the lifespan and efficiency of the battery.
Various methods have been proposed to solve the problems caused by using a composite material.
Japanese Patent Laid-Open Application No. 1994-318454 discloses an anode comprising a mixture of powder of metal flakes or alloy flakes, powder of carbon fiber flakes, and a binder. The invention includes a metal or an alloy powder that is laminated parallel to the surface of the electrode so that constant pressure is applied to the overall electrode against expansion/contraction of the electrode on operating the battery, thereby inhibiting insufficient charge collection caused by a repeating charge/discharge cycle. However, although powder flakes are used, it is difficult to solve the problems caused by charge/discharge by using only the simple mixture. Consequently, stress caused by expansion/contraction of metal and disconnection of electronic conduction path may worsen, thus reducing battery capacity with increasing the number of cycles of charge/discharge.
Japanese Patent Laid-Open Application No. 1997-249407 discloses an anode comprising graphite particles and metal particles that form an alloy with lithium. The anode is fabricated by preparing a raw powder including graphite particles and metal particles and pulverizing the powder to prepare graphite composites that have high crystalline graphite particles and metal microparticles in dispersed form. However, even in this case, it is difficult to avoid the binding of the metal particles with the graphite particles, since the invention uses a simple mixture method.
Japanese Patent Laid-Open Application No. 1998-003907 discloses an anode comprising carbon particles and metal microparticles that form an alloy with lithium, which are carried on the surface of the carbon particle. In this invention, the anode is formed not by simple mixing but by placing relatively small metal microparticles on the surface of the carbon particles. Thus, anode damage may be avoided since the difference in the relative volume expansion coefficient is minimized. In this invention, inventors assert that discharge capacity, charge/discharge rate and the like may improve. However, the problems such as stress caused by expansion/contraction and disconnection of electron transfer path still remain.
U.S. Pat. No. 6,589,696 discloses an anode formed by placing silicon microparticles on the surface of graphite particles and then coating them with a carbon film. This coating separates the metal and the electrolyte, and adheres the graphite material closely to silicon microparticles to block the reaction between the metal particles and electrolyte to improve conductivity. The anode material has good charge and discharge capacity and high charge/discharge efficiency. However, complete blocking between metal particles and an electrolyte may not be achieved with only the carbon film. Also, constant charge/discharge characteristics may be realized using another conducting agent, since the conductivity of the carbon film is lower than that of graphite.
Accordingly, there is a need for an anode active material with improved charge/discharge characteristics that can overcome the problems described above.